1. Field of the Invention
This invention relates to a driving force control system for a type of front-and-rear wheel drive vehicle that drives one pair of a pair of front wheels and a pair of rear wheels by an engine, and another pair of the pairs by an electric motor.
2. Description of the Prior Art
Conventionally, a driving force control system of the above-mentioned kind was proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 2000-79831. The front-and-rear wheel drive vehicle has front wheels thereof driven by an engine and rear wheels thereof driven by an electric motor. This driving force control system reduces the driving force for driving the front wheels for slip control, e.g. when the front wheels undergo a slip when the vehicle starts on a low-friction road surface, such as that of a snowy road. Further, under such a slip control of the front wheels, when it is determined e.g. from the vehicle speed that the vehicle is in a traveling condition in which the vehicle can move forward, the operation of the electric motor is inhibited, thereby saving electric energy of the vehicle.
The conventional driving force control system, however, simply inhibits the operation of the electric motor to thereby completely stop the assistance thereof, when it is determined during the slip control of the front wheels that the vehicle can move forward. Therefore, the total driving force for driving the vehicle tends to become short, and the slip of the front wheels is liable to be increased. Further, the determination as to whether the vehicle can move forward or not is carried out only by estimation based on the vehicle speed detected then, and therefore, depending on a subsequent operation of the accelerator pedal and the like, the slip of the front wheels can become excessively large, so that the front wheels cannot be maintained in an optimum slip condition, which makes it impossible to perverse traveling stability of the vehicle on a low-friction road surface.
Another driving force control system of the above-mentioned type was also proposed which causes the engine braking force to act on the front wheels and the braking torque caused by the electric motor to act on the rear wheels during a decelerating travel condition in which the accelerator pedal is released, to thereby brake the vehicle (e.g. Japanese Laid-Open Patent Publication No. 9-298802). In this case, the braking by the electric motor is carried out by producing a resisting force or drag force against the rotation of the rear wheels, during the decelerating travel of the vehicle.
According to the proposed driving force control system, however, when the decelerating travel is being carried out on a low-friction road surface, such as that of a snowy road, when a lateral force acts on the rear wheels by the user""s operation of the steering wheel, the rear wheels can loose their grip on the road surface to skid sideways, and in worst cases, the vehicle undergoes a spin. This is because the larger the braking force (braking torque) of the electric motor applied to the rear wheels, the smaller the lateral grip of the rear wheels on the road surface, and the deceleration causes the center of the gravity of the vehicle to be shifted forward to cause the axle load distribution to be unevenly shifted forward, whereby the lateral grip of the rear wheels is further reduced to make the above problem more conspicuous.
Further, the present assignee has already proposed another driving force control system of the above-mentioned type e.g. by Japanese Patent Application No. 11-366934. A front-and-rear wheel drive vehicles incorporating this control system has its front wheels driven by an engine via a torque converter, and its rear wheels driven by an electric motor. In this driving force control system, the driving conditions for driving the rear wheels by the electric motor, i.e. the conditions for executing the four-wheel drive include one determined based on a speed ratio of the torque converter. More specifically, the control system is configured such that if the detected speed ratio of the torque converter is equal to or larger than a predetermined value, the electric motor is stopped to execute the two-wheel drive, whereas if the former is lower than the latter, the electric motor is operated to execute the four-wheel drive. For instance, when the vehicle is started, the speed ratio tends to be small and hence the torque amplification factor of the torque converter is high, which makes the vehicle more prone to slippage. Therefore, the control system causes the electric motor to be operated to drive the rear wheels to assist the driving of the vehicle by the rear wheels, thereby improving the startability of the vehicle. Further, when the vehicle is being accelerated by the driver stepping on the accelerator pedal, the sliding of the torque converter increases, so that the speed ratio of the same temporarily drops. In such a case as well, the assistance of driving of the vehicle by the rear wheels is carried out to thereby ensure excellent acceleration of the vehicle. It should be noted that in the four-wheel drive state, first, the total driving force required for driving the vehicle is determined, and the driving force of the electric motor is determined such that the maximum output therefrom is an upper limit value thereof, and the driving force of the engine is determined as a difference obtained by subtracting the driving force of the electric motor from the total driving force.
In this driving force control system, however, there is room for improvement, because a torque step can be produced during acceleration for the following reason: In this driving force control system, as described above, when the vehicle is accelerated, a decrease in the speed ratio of the torque converter is expected which is caused by sliding of the torque converter, and the assistance of the electric motor is started on condition that the speed ratio becomes lower than the predetermined value. However, the sliding of the torque converter does not occur instantly, but progressively increases, so that there is a time lag before the speed ratio actually decreases below the predetermined value, i.e. before the assistance of the electric motor is started. On the other hand, the total driving force continues to be increased in response to the demand for acceleration, and since the driving force of the electric motor remains set to 0, the driving force of the engine continues to be increased at the same ratio. As a result, when the speed ratio becomes lower than the predetermined value, a large driving force of the electric motor is suddenly generated, and the driving force of the engine suddenly drops by an extent equal to the large driving force of the electric motor. In this case, it is difficult to synchronize the generation of the driving force of the electric motor and the drop of the driving force of the engine, so that occurrence of a certain amount of torque step is inevitable. Particularly, when the vehicle is accelerated from a low-speed traveling condition, or from a decelerating condition, the total driving force is largely increased even when the accelerator pedal is stepped on by a small amount, and therefore, the above-mentioned problem tend to be conspicuous.
It is a first object of the invention to provide a driving force control system for a front-and-rear wheel drive vehicle, which is capable of properly controlling the driving force of an engine without inhibiting the assistance of an electric motor when drive wheels driven by the engine undergo a slip, such that an optimum slip condition of the drive wheels can be ensured even on a low-friction road surface, thereby making it possible to preserve traveling stability of the vehicle.
It is a second object of the invention to provide a driving force control system for a front-and-rear wheel drive vehicle, which enables rear wheels of the vehicle to have an appropriate lateral grip even when the driver operates the steering wheel when the vehicle is performing decelerating travel on a low-friction road surface or a downhill slope, thereby making it possible to preserve traveling stability of the-vehicle.
It is a third object of the invention to provide a driving force control system for a front-and-rear wheel drive vehicle that enables the assistance of an electric motor to be smoothly performed without developing a torque step when the vehicle is accelerated, thereby ensuring an excellent acceleration and drivability.
To attain the above first to third objects, the present invention provides a driving force control system for a front-and-rear wheel drive vehicle that drives one pair of respective pairs of front drive wheels and rear drive wheels by an engine, and another pair of the respective pairs by an electric motor, the driving force control system comprising:
vehicle speed-detecting means for detecting a vehicle speed of the vehicle;
driving force demand degree-detecting means for detecting a degree of demand for a driving force for driving the vehicle;
target driving force-calculating means for calculating a target driving force for driving the vehicle, based on at least the vehicle speed and the degree of demand for the driving force;
traveling condition-determining means for determining a present traveling condition of the vehicle; and
driving force control means for controlling a driving force of the engine and a driving force of the electric motor based on the calculated target driving force, in dependence on the traveling condition of the vehicle determined by the traveling condition-determining means.
According to this driving force control system, a target driving force is calculated based on the detected vehicle speed and the degree of demand for a driving force for driving the vehicle, and the present traveling condition of the vehicle is determined. The driving force of the engine and that of the motor are controlled based on the calculated target driving force in dependence on the traveling condition of the vehicle. This makes it possible to ensure a stable travel, and an excellent acceleration and drivability.
Particularly to attain the first object, it is preferred that the driving force demand degree-detecting means comprises accelerator opening-detecting means for detecting an accelerator opening, the traveling condition-determining means including differential rotational speed-detecting means for detecting a differential rotational speed between a rotational speed of the front drive wheels and a rotational speed of the rear drive wheels based on a parameter indicative of the traveling condition of the vehicle, target differential rotational speed-setting means for setting a target differential rotational speed based on the parameter indicative of the traveling condition of the vehicle, and slip determination means for determining a slip condition of the one pair driven by the engine, based on the detected differential rotational speed and the target differential rotational speed, the driving force control means including target motor driving force-calculating means for calculating a target motor driving force of the electric motor based on the target driving force, target engine driving force-calculating means for calculating a target engine driving force of the engine based on the target driving force and the target motor driving force, motor drive control means for controlling driving of the electric motor based on the target motor driving force, engine drive control means for controlling driving of the engine based on the target engine driving force, and engine driving force-correcting means for decreasing the target engine driving force such that the differential rotational speed is held at the target differential rotational speed, when it is determined by the slip determination means that the one pair driven by the engine are slipping.
According to this preferred embodiment, based on the detected accelerator opening, the target driving force for driving the vehicle is calculated, and based on the target driving force, the target driving force of the electric motor is calculated. Further, based on the target driving force for driving the vehicle and the target driving force of the electric motor, the target driving force of the engine is calculated. Further, the differential rotational speed between the rotational speed of the front drive wheels and that of the rear drive wheels is detected, and a target differential rotational speed is set based on a parameter indicative of the traveling condition of the vehicle. Then, based on the detected differential rotational speed and the target differential rotational speed, a slip condition of the drive wheels driven by the engine is determined, and when it is determined that these drive wheels are slipping, the target engine driving force is decreased such that the differential rotational speed is held at the target differential rotational speed.
As described above, according to this preferred embodiment, whether or not the drive wheels driven by the engine are slipping is determined based on the actual differential rotational speed between the rotational speed of the front wheels and that of the rear wheels, and when the drive wheels driven by the engine are slipping, the driving force of the engine is decreased to hold the actual differential rotational speed at the target differential rotational speed. In other words, when the drive wheels driven by the engine undergo a slip, the driving force of the engine is properly controlled such that the differential rotational speed is held at the target differential rotational speed without stopping the assistance of the electric motor, so that even when the vehicle is running on a low-friction road surface, the drive wheels driven by the engine can be maintained at an optimum slip condition, whereby stable traveling can be ensured.
More preferably, the driving force control means further includes motor driving force-correcting means for increasing the target motor driving force when the slip determination means determines that the one pair driven by the engine are slipping.
According to this preferred embodiment, when the drive wheels driven by the engine are slipping, the target engine driving force is decreased and at the same time, the target motor driving force is increased whereby the differential rotational speed can be promptly converged to the target differential rotational speed.
More preferably, the traveling condition-determining means further includes differential rotational speed change amount-detecting means for detecting an amount of change in the differential rotational speed, and the engine driving force-correcting means decreases the target engine driving force according to the detected amount of change in the differential rotational speed.
According to this preferred embodiment, the target driving force of the engine is decreased when the drive wheels are slipping, according to the amount of change in the differential rotational speed, so that the convergence of the differential rotational speed to the target differential rotational speed can be enhanced.
For instance, the parameter indicative of the traveling condition of the vehicle includes at least one of a degree of a slope of a road, a steering angle, the vehicle speed, and the accelerator opening.
According to this preferred embodiment, it is possible to properly set the target differential rotational speed according to the actual traveling condition of the vehicle and the driver""s intention.
Particularly to attain the second object, it is preferred that the vehicle includes an accelerator pedal, and a steering wheel, the driving force demand degree-detecting means including accelerator condition-detecting means for detecting whether or not the accelerator pedal is in a released condition, the traveling condition-determining means including downhill traveling-determining means for determining whether or not the vehicle is traveling downhill, and steering angle-detecting means for detecting an steering angle of the steering wheel, the driving force control means including target deceleration-setting means for setting a target deceleration based on the detected steering angle when the accelerator condition-detecting means detects that the accelerator pedal is in the released condition and at the same time the downhill traveling-determining means determines that the vehicle is traveling downhill, engine braking force-calculating means for calculating an engine braking force according to the detected vehicle speed when the accelerator condition-detecting means detects that the accelerator pedal is in the released condition, target braking force-setting means for setting a target braking force of the electric motor for braking the rear wheels, based on the set target deceleration and the calculated engine braking force, and drive control means for controlling driving of the electric motor based on the set target driving force.
According to this preferred embodiment, when the released condition of the accelerator pedal is detected and at the same time it is determined that the front-and-rear drive vehicle is traveling downhill, the target deceleration is set based on the detected steering angle, and when the released condition of the accelerator pedal is detected, the engine braking force is calculated based on the detected vehicle speed. Then, based on the set target deceleration and the calculated engine braking force, the target braking force of the electric motor for braking the vehicle is set. In this case, since the target braking force of the electric motor is set based on the target deceleration and the engine braking force, as the target deceleration, i.e. the deceleration of the whole vehicle is larger, the target braking force of the electric motor is set to a larger value. Then, based on the set target braking force, the driving of the electric motor is controlled. This makes it possible to set the target deceleration to an appropriate value responsive to the steering angle of the vehicle traveling downhill. Therefore, when the steering angle is turned through a larger steering angle, the target deceleration, i.e. the deceleration of the whole vehicle can be set to a smaller value, to thereby reduce the braking force applied to the rear wheels and accordingly prevent the biased distribution of the axle load toward the front wheel side. As a result of the reduced braking force applied to the rear wheels, and the prevention of the biased distribution of the axle load toward the front wheel side, it is possible to enhance the lateral grip of the rear wheels on the road surface and thereby positively control a skid of the vehicle even when the lateral force acts on the rear wheels through turning of the steering wheel during downhill traveling of the vehicle on a low-friction road surface. As a result, it is possible to ensure a stable traveling of the vehicle.
More preferably, the vehicle includes a brake, the traveling condition-determining means including brake operation-detecting means for detecting whether or not the brake is being operated, the driving force control means further including target deceleration-increasing means for correcting the set target deceleration such that the set target deceleration is increased to a larger value than assumed when the brake is not being operated, when the brake operation-detecting means detects that the brake is being operated.
According to this preferred embodiment, when the brake is being operated, the target deceleration is increased to a larger value than assumed when the brake is not being operated. That is, when the rear wheels are braked by the driver""s operation of the brake, the target deceleration is made larger than when the driver is not carrying out the braking operation, whereby the braking force applied to the whole vehicle can be increased according to the driver""s intention.
More preferably, the vehicle includes clutch means for disconnecting and connecting between the rear wheels and the electric motor, the driving force control means including clutch driving means for disconnecting the clutch means when the vehicle speed is higher than a predetermined vehicle speed, and disconnecting the clutch means when the vehicle speed is equal to or lower than the predetermined vehicle speed, and target deceleration-decreasing means for correcting the set target deceleration such that the set target deceleration is decreased to a smaller value than the target deceleration increased by the target deceleration-increasing means.
According to this preferred embodiment, the clutch means for disconnecting and connecting between the rear wheels and the electric motor is disconnected when the vehicle speed is larger than a predetermined vehicle speed, and connected when the same is equal to or lower than the predetermined vehicle speed. Further, when the clutch means is disconnected and at the same time the operation of the brake is detected, the target deceleration is reduced to a smaller value than the target deceleration increased by the correction. With reduction of the target deceleration, the braking force of the electric motor is set to a reduced value, so that when the clutch means is connected in response to a decrease of the vehicle speed to a predetermined value or lower, the small braking force of the electric motor is applied to the rear wheels. This can prevent a sudden application of a larger braking force of the electric motor to the rear wheels, to thereby prevent a braking shock or a slightly locked state of the wheels on a low-friction road surface.
Particularly to attain the second object, it is preferred that the vehicle include an accelerator pedal and a steering wheel, the driving force demand degree-detecting means including accelerator condition-detecting means for detecting whether or not the accelerator pedal is in a released condition, the traveling condition-determining means including steering angle-detecting means for detecting a steering angle of the steering wheel, the driving force control means including engine braking force-calculating means for calculating an engine braking force of the engine according to the detected vehicle speed when the accelerator condition-detecting means detects that the accelerator pedal is in the released condition, target braking force-setting means for setting a target braking force of the electric motor for braking the rear wheels to a value corresponding to the calculated engine braking force, target braking force-correcting means for correcting the set target braking force according to the detected steering angle, and drive control means for controlling driving of the electric motor based on the corrected target braking force.
In general, when the front-and-rear wheel drive vehicle is braked, the braking force of the front wheels and that of the rear wheels are set to respective values equal to each other, whereby the behavior of the vehicle becomes hard to get out of order but stable. Therefore, according to the preferred embodiment, when the released condition of the accelerator pedal is detected, the engine braking force is calculated according to the detected vehicle speed, and the target braking force of the electric motor is set to a value equal to the calculated engine braking force, whereby the behavior of the vehicle in decelerating travel by release of the accelerator pedal can be stabilized. Further, since the target braking force is corrected according to the detected steering angle, the target deceleration can be set to an appropriate value reflecting the steering angle of the vehicle assumed during the decelerating travel. This enables the braking force of the electric motor to be made smaller as the steering wheel is turned through a larger steering angle, whereby through the reduction of the braking force applied to the rear wheels and the prevention of the biased distribution of the axle load toward the front wheel side, the lateral grip of the rear wheels on the road surface can be enhanced. As a result, even when the vehicle is performing decelerating travel on a low-friction road surface, for instance, it is possible to positively suppress a skid of the rear wheels caused by the operation of the steering wheel. (It should be noted that xe2x80x9cvalues equal to each otherxe2x80x9d is used to mean not only values quite identical to each other but also values in a substantially equal range.)
Particularly to attain the third object, it is preferred that the vehicle includes a torque converter, and is driven while switching between a four-wheel drive mode in which the one pair are driven by the engine via the torque converter and at the same time the another pair are driven by the electric motor and a two-wheel drive mode in which the driving of the another pair by the electric motor is inhibited, the driving force demand degree-detecting means including accelerator opening-detecting means for detecting an accelerator opening, the traveling condition-determining means including speed ratio-detecting means for detecting a speed ratio of the torque converter, reference speed ratio-storing means for storing a predetermined reference speed ratio, and speed ratio-determining means for determining whether or not the detected speed ratio is smaller than the predetermined reference speed ratio, the driving force control means including target motor driving force-calculating means for calculating a target motor driving force of the electric motor based on the target driving force, target engine driving force-calculating means for calculating a target engine driving force of the engine based on the target driving force and the target motor driving force, motor drive means for driving the electric motor based on the target motor driving force, and motor drive-permitting means for permitting the driving of the electric motor by the motor drive means when an amount of change in the target driving force is equal to or larger than a predetermined value, and when the amount of change in the target driving force is smaller than the predetermined value, permits the driving of the electric motor on condition that the speed ratio-determining means determines that the detected speed ratio is smaller than the predetermined reference speed ratio.
According to this preferred embodiment, based on the detected accelerator opening and vehicle speed, a target driving force for driving the vehicle is calculated, and based on the calculated target driving force, a target motor driving force of the electric motor is calculated. Further, based on the vehicle target driving force and the target motor driving force thus calculated, a target engine driving force is calculated. Based on the target motor driving force, the motor drive means drives the electric motor to drive the other pair of the drive wheels, the vehicle is driven in a four-wheel drive state in which the driving of the vehicle is assisted by the electric motor. Further, the speed ratio of the electric motor is detected and comparison between the detected speed ratio and a predetermined speed ratio is performed. Motor drive-permitting means permits the driving of the electric motor by the motor drive means when the amount of change in the calculated target driving force is equal to or larger than a predetermined value, whereas the driving of the electric motor is permitted on condition that the detected speed ratio is smaller than the predetermined reference speed ratio when the amount of change in the calculated target driving force is smaller than the predetermined reference value.
As described above, according to this preferred embodiment, the amount of change in the target driving force for driving the vehicle is equal to or larger than the predetermined value, the driving of the electric motor is permitted irrespective of the speed ratio of the torque converter, thereby enabling execution of the assisting mode for assisting the driving of the vehicle by the electric motor. Therefore, e.g. when the vehicle is accelerated from a low-speed or decelerating condition, the assistance of the electric motor is instantly started without waiting for the speed ratio of the torque converter to become lower than the predetermined reference speed ratio and hence without being influenced by the delay of a change in the speed ratio of the torque converter. Therefore, it is possible to start the assistance of the electric motor from a state in which the motor driving force is still small, and progressively increase the motor driving force, and progressively reduce the engine driving force according to the progressive increase in the motor driving force, whereby the assistance of the electric motor can be performed without developing a torque step.
On the other hand, if the amount of change in the target driving force is smaller than the predetermined value, i.e. influence on the torque step is small, the driving of the electric motor is permitted on condition the speed ratio of the torque converter is smaller than the reference speed ratio, whereby it is possible to properly perform the assistance of the electric motor according to the actual speed ratio. This makes it possible to ensure excellent acceleration and drivability.
Also, particularly to attain the third object, it is preferred that the vehicle includes a torque converter, and is driven while switching between a four-wheel drive mode in which the one pair are driven by the engine via the torque converter and at the same time the another pair are driven by the electric motor and a two-wheel drive mode in which the driving of the another pair by the electric motor is inhibited, the driving force demand degree-detecting means including an accelerator opening-detecting means for detecting an accelerator opening, the traveling condition-determining means including speed ratio-detecting means for detecting a speed ratio of the torque converter, reference speed ratio-storing means for storing a predetermined reference speed ratio, speed ratio-determining means for determining whether or not the detected speed ratio is larger than the predetermined reference speed ratio, and stepping operation-detecting means for detecting whether or not the accelerator pedal in the released condition is stepped on, during the traveling of the vehicle, the driving force control means including target motor driving force-calculating means for calculating a target motor driving force of the electric motor, motor drive means for driving the electric motor based on the calculated target motor driving force, and motor drive-permitting means for permitting the driving of the electric motor by the motor drive means when the stepping operation-detecting means detects that the accelerator pedal is stepped on, and when the stepping operation-detecting means does not detect that the accelerator pedal is stepped on, inhibits the driving of the electric motor on condition that the speed ratio-determining means determines that the detected speed ratio is smaller than the predetermined reference speed ratio.
According to this preferred embodiment, when it is detected whether or not the accelerator pedal in the released state is stepped during traveling of the vehicle. If such an operation of stepping on the accelerator pedal is detected, the driving of the electric motor is permitted, whereas if not, the driving of the electric motor is inhibited on condition that the speed ratio of the torque converter is larger than the predetermined reference speed ratio.
As described above, if the vehicle is accelerated by stepping on the accelerator pedal when the vehicle is in a decelerating condition in which the accelerator pedal is released, the driving of the electric motor is permitted irrespective of the speed ratio of the torque converter. In this case, the assistance of the electric motor can be started immediately after the start of the acceleration without being affected by the delay in a change in the speed ratio, and the motor driving force can be progressively increased starting with a small value. Therefore, the assistance of the electric motor can be smoothly performed without developing a torque step caused by the delay in a change in the speed ratio of the torque converter.